/**
 * Licensed to the Apache Software Foundation (ASF) under one or more contributor license
 * agreements.  See the NOTICE file distributed with this work for additional information regarding
 * copyright ownership.  The ASF licenses this file to you under the Apache License, Version 2.0
 * (the "License"); you may not use this file except in compliance with the License.  You may obtain
 * a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software distributed under the License
 * is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express
 * or implied. See the License for the specific language governing permissions and limitations under
 * the License.
 */

package org.apache.zookeeper.server.persistence;

import org.apache.jute.Record;
import org.apache.zookeeper.txn.TxnHeader;

import java.io.IOException;

/**
 * 读取事务日志的接口
 *
 */
public interface TxnLog {

    /**
     * roll the current log being appended to
     *
     * @throws IOException
     */
    void rollLog() throws IOException;

    /**
     * Append a request to the transaction log
     * @param hdr the transaction header
     * @param r the transaction itself
     * returns true iff something appended, otw false 
     * @throws IOException
     */
    boolean append(TxnHeader hdr, Record r) throws IOException;

    /**
     *
     * 从给定的事务id，开始读取事务日志[Start reading the transaction logs from a given zxid]
     *
     * @param zxid 事务id
     * @return 返回：读取日志中下一个事务的迭代器。 [returns an iterator to read the next transaction in the logs.]
     * @throws IOException
     */
    TxnIterator read(long zxid) throws IOException;

    /**
     * 获取最后的已经记录的事务id
     *
     * @return 事务id
     * @throws IOException
     */
    long getLastLoggedZxid() throws IOException;

    /**
     * truncate the log to get in sync with the leader.
     * @param zxid the zxid to truncate at.
     * @throws IOException
     */
    boolean truncate(long zxid) throws IOException;

    /**
     * the dbid for this transaction log. 
     * @return the dbid for this transaction log.
     * @throws IOException
     */
    long getDbId() throws IOException;

    /**
     * commmit the trasaction and make sure
     * they are persisted
     * @throws IOException
     */
    void commit() throws IOException;

    /**
     * close the transactions logs
     */
    void close() throws IOException;

    /**
     * an iterating interface for reading 
     * transaction logs. 
     */
    interface TxnIterator {
        /**
         * return the transaction header.
         * @return return the transaction header.
         */
        TxnHeader getHeader();

        /**
         * return the transaction record.
         * @return return the transaction record.
         */
        Record getTxn();

        /**
         * go to the next transaction record.
         * @throws IOException
         */
        boolean next() throws IOException;

        /**
         * close files and release the 
         * resources
         * @throws IOException
         */
        void close() throws IOException;
    }
}

